The present invention relates to an optical transmitter outputting optical-frequency division-multiplexed (optical FDM) signals and an optical transmission apparatus transmitting optical FDM signals and in particular to an optical transmitter and an optical transmission apparatus suitable for realizing communication of large capacity through one optical fiber by narrowing the frequency spacing (channel spacing) between different signals constituting the optical-frequency-division-multiplexed signals.
In an optical FDM transmission system, by which a plurality of signals are transmitted through one optical fiber by the optical frequency division multiplexing, the channel spacing is an important parameter determining the transmission capacity. Heretofore it is thought that this channel spacing is limited by crosstalk from adjacent signals (adjacent channels). For example, in a literature "Institute of Electronics, Information and Communication Engineers, Technical Report, Optical Communication System Study Group OCS 89-31, Jun. 23, 1989" (in Japanese) (hereinbelow called simply "Literature 1"), the channel spacing is studied in detail both experimentally and theoretically.
The crosstalk from adjacent channels is, e.g. in the case where the heterodyne detection is used, a phenomenon that a part of the spectrum of an adjacent channel converted into an intermediate frequency (IF) signal as an image enters in the passband of an IF filter for extracting a signal to be received (received channel), which gives rise to a problem, because the enters component acts as if it were noise. FIGS. 2A to 2D show an example of the allocation of the spectrum of an optical-frequency-division-multiplexed signal in an optical frequency division multiplexing system (refer to Literature 1), in which the frequency shift keying (FSK) modulation is used for the modulation scheme and a single filter and envelope detector are used for the demodulation scheme. FIG. 2A indicates allocation of spectrums of respective channel (respective ch.) on the abscissa on the axis of the optical frequency. The number of multiplexed signals (total number of channels) is N. Mark n denotes the channel number and fulfils 1&lt;n.ltoreq.N. D.sub.op represents the channel spacing and .DELTA.f the frequency deviation. Further m and s represent the mark and the space component of the signal, respectively. LO indicates the power from the local oscillator light source. FIG. 2B indicates the spectrum of the signal obtained by heterodyne-detecting this optical-frequency-division-multiplexed signal. As the result of the heterodyne detection, the channels are allocated on the IF frequency axis in the order of n, (n-1), (n+1), ....., from the low frequency side. The IF filter (the transmission characteristics thereof being represented by a dot-dashed line) is so set that only the mark component (central frequency: f.sub.m) of the received channel (channel n ch.) is extracted, as indicated in FIG. 2C. FIG. 2D shows the output signal from the IF filter. The value of the channel spacing D.sub.if from the adjacent channel on the IF frequency axis is determined by the crosstalk from the adjacent channel (n-1). The channel spacing D.sub.if is discussed in detail in Literature 1. At this time the channel spacing D.sub.op on the optical frequency axis is given by the following equation; EQU D.sub.op =D.sub.if +2.multidot.f.sub.m +.DELTA.f (1)
Heretofore it was not possible to decrease the channel spacing on the optical frequency axis to a value below the frequency spacing given by Eq (1).